The present invention relates to a power converter in which the AC side is connected through a reactor to an AC power source while the DC side is connected to a smoothing capacitor and a load, and more particularly a control device which controls the power converter of the described type by means of the pulse-width-modulation method so that the power converter is able to be used not only as a power rectifier but also as a power inverter.
In case of a power converter composed of a diode bridge for converting AC from a commercial power supply into DC which is applied to a load, a system voltage is varied due to the generation of the reactive power on the commercial power supply side and higher harmonics are generated so that a communication line such as a telephone line extended adjacent to a power transmission line is subjected to inductive interference.
In order to solve the above-described problems, there has been already proposed and used in practice a power converter called a PWM converter of the type in which the converter is composed of a GTO (Gate Turn-Off thyristor) or a thyristor with a forced commutation circuit and the pulse-width-modulation system is used to control such switching elements.
The conventional control device of the type described has the features that regardless of the direction of power flow between the AC and DC sides, that is, regardless of the power running for transmitting power from the AC side to the DC side and the regenerative running for transmitting power from the DC side to the AC side, the power factor of the power supply on the AC side can be always maintained at "1" and that the higher harmonics can be minimized because the AC current Is becomes sinusoidal.
Meanwhile, the microprocessors have been remarkably developed recently so that there is a growing tendency to substitute the microprocessors for analog circuits in order to make the devices accomplish various functions with a high accuracy. The same tendency has been observed in the field of controlling the power converters. However, when the microprocessor is used in the pulse-width-modulation control system, it is difficult to employ a conventional control system shown in FIG. 22 as a control algorithm.
That is, in order to obtain the features of the conventional control system shown in FIG. 22, the operation of an instantaneous-value reference signal Vcr for the voltage on the AC side of a power converter 3 must be carried out at a period of at least several tens microseconds. However, when the microprocessor is employed in order to precisely carry out the function of the conventional control system, a time interval for carrying out an arithmetic operation becomes considerably longer so that the period at which the instantaneous command is operated becomes a few milliseconds. As a result, the desired function cannot be obtained.